The present invention relates to tunable microwave filter apparatus and methods and, more particularly, to such apparatus and methods employing high temperature superconductors (HTS) and to radar receivers employing such filters.
In radar applications, preselector filtering is desirable to achieve full dynamic radar range in the presence of a multi-signal environment. Switched filter-banks have been under development for this purpose.
Conventional filter banks are unable to provide the narrow, (&gt;100 MHz) bandwidth required in a compact, cost effective manner. Superconducting filter bands which can achieve less than 100 MHz bandwidths have been demonstrated. However, this filter technology requires a complex and bulky cryogenic module, since a large number of megahertz channels (i.e., 50.times.20 MHz channels) are required to cover a 1 GHz bandwidth. GaAs switches provide switching, but cause most of a 1 dB loss in these banks and further generate much of the heat which requires cryogenic cooling.
Tunable bandpass filters, which trace the frequency of a hopping or scanning radar signal, could significantly reduce the size, complexity, and power losses associated with switched-bank preselector filters. Various technologies have been employed in designing tunable bandpass filters for microwave applications, but all such known filters have disadvantages associated with them.
For example, tunable YIG filters employ magnetic field tuning. However, these filters are slow in tuning, require a continuous current to supply the magnetic bias field resulting in significant power dissipation, and cannot handle higher power levels (i.e., restricted to about 100 milliwatts or less).
Mechanical filters employ tunable waveguide structure with mechanical element motion needed for tuning. Such filters are bulky and inherently tune slowly.
Varactors employ reverse biased diodes to provide fast tuning, but operate with nonlinearity and cannot handle power above about 1 milliwatt.
Ferroelectric filters provide tuning by using electric field control to vary the dielectric constant characteristic of the filter. Ferroelectric filters tune rapidly, but operate nonlinearly and cannot handle power above about 1 milliwatt.
For further information on tunable microwave filters, reference is made to TUNABLE MICROWAVE AND MILLIMETER-WAVE BAND-PASS FILTERS, Jaroslaw Uher and Wolfgang J. R. Hoefer, IEEE Transactions on MTT, Volume 39, Pages 643-653, 1991.
In summary, the known prior art tunable microwave filters lack the combination of rapid (microsecond) tuning, narrow band-pass, low insertion loss, and good linearity needed for efficient radar preselector filtering and similar microwave applications. In turn, radar receivers have essentially been restricted in dynamic range performance due to the unavailability of needed, practical preselector filters.